An ad hoc wireless network is a wireless network which enables wireless communication devices to directly communicate with each other, and enables wireless communication devices being so apart from each other that radio waves do not directly reach between them to let a wireless communication device existing between them relay the radio waves so as to carry out mutual communication. Respective nodes which are wireless communication devices are autonomously formed in the ad hoc wireless network. Each of the nodes carries out a process as a router so as to enable a relay between the nodes. Incidentally, the ad hoc wireless network is called a multi-hop wireless network, as well.
Reactive and proactive protocols are known as routing protocols for controlling a communication path for a data relay in the ad hoc wireless network. According to the reactive protocol, a communication path is searched and determined when a communication session starts. According to the proactive protocol, messages are exchanged between adjacent nodes before a communication session starts so that a communication path is determined in advance. The adjacent nodes exchange the messages in accordance with the proactive protocol by, e.g., broadcasting a control packet such as a HELLO packet.
Various methods have been proposed for the routing protocol, i.e., a method for communication path control, of the ad hoc wireless network. It is known, e.g., that the nodes each hold connection probability indicating connectivity to their adjacent nodes, and change the communication path based on a change of the connection probability. The connection probability is calculated by a signal-to-noise ratio on an adjacent link with an adjacent node, etc.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2007-243575
Incidentally, according to an ordinary method for communication path control in the ad hoc wireless network, quality of a link that reaches an addressed node is measured every regular period of time, e.g., every period for transmitting a broadcast packet. Then, a path is selected from a plurality of path candidates which reach the addressed node based on an average value of link quality of the respective path candidates for the regular period of time. A try at selecting a path based on an average value of link quality for a regular period of time, however, may possibly cause improper path selection in a case where the link quality between the adjacent nodes drops in an instant. That is, if the link quality with an adjacent node on a path selected owing to a high average value of the link quality drops a lot in an instant, data is not correctly transmitted to the adjacent node and retransmission of the data occurs. Thus, a period of time required for the data communication between the nodes is extended.